This invention concerns the removal of thermal barrier coatings from components that are typically required to operate in a high temperature environment in use.
There exist a number of applications in which metallic components are required to operate at elevated temperatures in use, such as, for example, within combustion engines. Exposure of components to prolonged and/or intermittent heating loads can degrade the components and reduce their operational life as well as increasing the risk of unexpected component failure. For some applications, such as within the high pressure turbine of a gas turbine engine, aerofoil components are required to operate at temperatures above their melting point.
Thermal Barrier Coatings (TBC) typically comprise a ceramic material with low thermal conductivity and are used to insulate the underlying metallic structural components from the high temperature operational environment and thereby reduce degradation by oxidation and thermal fatigue.
In some other applications, rather than reducing the maximum temperature to which a component is exposed, TBCs are used to prevent heat loss and/or material loss (e.g. by oxidation) from metal components in use. Such uses are typically aimed at improving thermal efficiency in elevated temperature environments that may be within or beyond the operating range of the metal components.
Conventional ceramic TBCs may be applied by a number of different techniques, such as physical vapour deposition or spray coating techniques, to a metal substrate. Either complete or partial coatings may be applied dependent on the intended application of the substrate. The aim of the coating process is typically to achieve a very strong bond between the metal substrate and the ceramic coating layer (e.g. using a bond coat on the substrate). This strong bond is essential in order for the coating to be able to withstand repeated thermal loading cycles over its operational life as well as any other physical loading, including stress/strain and possible impact forces, placed on the component due to its intended function.
However there is also a need to remove the existing TBC from a component as part of a component salvage and/or repair process. Ideally, the coating is removed in its entirety without affecting the underlying component such that a new TBC can be reliably applied to the component. A conventional TBC removal method involves water jet stripping. This is a ‘line-of-sight’ process and requires careful control of the relative movement between a high power water jet and the component in order to completely remove the coating. There exists the possibility that small remnants of the TBC may not be removed, which can cause issues for salvage or repair of the component. For some component geometries, there exists a problem in removing a complete coating using a line-of-sight removal process. That is to say, some regions of a component can be hidden and inaccessible to a water jet. One example of such a problem occurs with components comprising multiple aerofoils/vanes, such as nozzle guide vanes for a gas turbine engine, in which a trailing portion of an upstream vane shadows a leading portion of a downstream vane.
Furthermore a water jet stripping process can only accommodate a single component at a time.
There has now been devised a novel process which overcomes or substantially mitigates some or all of the above-mentioned and/or other disadvantages associated with the prior art.